(1) Field of the Invention
The present invention relates to a method of measuring a distribution of crystal grains in metal sheet and an apparatus therefor. More particularly, the present invention relates to a highly practical method of measuring a distribution of crystal grains in metal sheet and an apparatus therefor, suitable for use in producing an anisotropic silicon steel sheet high in electromagnetic properties in a rolling direction, capable of non-destructively measuring the distribution of crystal grains in orientation of the anisotropic silicon steel sheet on line.
(2) Prior Art
For the purpose of applying it to a core of an AC step-up transformer and the like, there has been produced an anisotropic silicon steel sheet high in electromagnetic properties in the rolling direction. This anisotropic silicon steel sheet is produced by obtaining the crystal grains directed in a so-called GOS orientation, in which a plane {110} is parallel to the sheet surface and a direction &lt;100&gt; is parallel to the rolling direction.
The larger the number of the grains close to the GOS orientation is, the higher the electromagnetic properties of the anisotropic silicon steel sheet are. However, in the actual manufacturing process, it is not necessarily possible to produce only the grains close to the GOS orientation. Due to a change in the conditions of manufacture or a disturbance, grains greatly shifted from this GOS orientation, i.e. so-called abnormal grains are formed.
FIG. 14 shows a typified view of a product 10 including the abnormal grains 10A. A normal grain 10B close to the GOS orientation has a grain diameter as large as several mm-several cm, however, the abnormal grain 10A has a grain diameter of several mm or less in general, the orientation of the grain is directed in random directions, and is distributed such that it extends generally in the rolling direction as shown in FIG. 14.
In the manufacturing process of the above-described anisotropic silicon steel sheet, as instruments for detecting the electromagnetic properties of the product, an Epstein detector and a single sheet tester are used off line, while a continuous iron loss tester is used on line, thus measuring an iron loss of the steel sheet and a density of magnetic fluxes. However, the aforesaid measuring instruments can measure only the mean electromagnetic properties in the widthwise direction of the steel sheet, but cannot measure the distribution of the abnormal grains 10A.
On the other hand, as a method of measuring the distribution of the abnormal grains of the silicon steel sheet, there is a macro-etch method, wherein the product is cut into strip-like forms, an insulation coating film on the surface thereof is peeled off and the surface is etched by use of a Nital etchant, whereby the fact that etched effects are different in accordance with the orientations of the crystal grains is utilized, so that the distribution of the crystal grains is observed.
However, application of this macro-etch method is limited to off line, but cannot be applied to on line. Moreover, it is necessary to cut the product into the strip-like forms. Further, etching can be made only after the insulation coating film is peeled off. Thus, much labor is needed, and there is the problem of handling such as a treatment of chemicals. In addition, since this is a destructive test and a sampling inspection, the quality of the product over the total length thereof cannot be ensured. Further, such a problem is presented that the efficiency is low and so forth from the viewpoint of the type of working.
On the other hand, as techniques relating to the present invention, there has been proposed a method of non-destructively testing the characteristic of a steel sheet by the utilization of propagation characteristics of ultrasonic wave in the crystal.
More specifically, in general, a sonic speed v of a longitudinal ultrasonic wave propagated in a direction of &lt;n.sub.1, n.sub.2, n.sub.3 &gt; of a body of isometric crystal is given as a root of the following equation. ##EQU1## where n.sub.1, n.sub.2, and n.sub.3 are direction cosines between the propagated direction and the principal axis of crystal, and a, b and c are shown in the following equations, respectively. EQU a=C.sub.11 -C.sub.44 ( 2) EQU b=C.sub.12 +C.sub.44 ( 3) EQU c=C.sub.11 -C.sub.12 -2C.sub.44 ( 4)
where Cij is an ij component of an elastic matrix.
Accordingly, when the body is steel, and an elastic constant of steel is given, whereby a sonic speed v of the longitudinal ultrasonic wave propagated in a single crystal in various directions can be calculated from the equation (1).
FIG. 15 shows examples of calculated results in forms on a stereo-projection drawing. Because the sonic speed v is different in accordance with the propagated directions of the ultrasonic wave, the sonic speed v is measured, so that the orientation of the crystals can be determined to a certain extent. For example, when the direction of the sheet thickness is given as the Z-axis, a sonic speed v in the direction of the Z-axis is measured and, when the measured value is 6500 m/s, it can be said that, in this body, an axis [111] of the crystal is laid in the direction of the sheet thickness.
As the conventional example of the measurement of the characteristics of a material by the utilization of the characteristics of the ultrasonic wave propagated in a crystal as described above, there is a method of discriminating a cast structure by the ultrasonic wave as disclosed in Japanese Patent Unexamined, Publication No. 126992/1978. According to this method, a propagation time of the ultrasonic wave in the direction of the sheet thickness of a poly-crystalline body is measured, and from this measured value, a thickness of a columnar crystal and a thickness of an isometric crystal are estimated.
When a sonic speed in the direction of sheet thickness of a body is to be measured in general, as shown in FIG. 16, ultrasonic wave is directed toward a body 11 to be measured in the direction of sheet thickness by use of an ultrasonic probe 12, a time interval t of a row of bottom echoes B1, B2 and B3 which are produced thereby as shown in FIG. 17, for example, is measured, and the sonic speed v is calculated by the following equation. EQU v=2d/t (5)
where d indicates a sheet thickness of the body 11. In addition, S denotes a signal produced by a surface reflected wave.
However, also in this method, when the sheet thickness reaches a length close to a wavelength of the ultrasonic wave, B1, B2 and B3 in the row of bottom echoes overlie one another, the time interval t cannot be measured.
For example, if a frequency of the ultrasonic wave is 10 MHz, the wavelength of the longitudinal wave is about 0.6 mm, while the sheet thickness d of the silicon steel sheet 11 to be measured is 0.1-0.5 mm, then this method is not applicable.
In order to apply this method, it is necessary that the frequency of the ultrasonic wave be 100 MHz or thereabove and short pulses are adopted, whereby the problems of electric circuits, largely attenuated ultrasonic wave and the like are raised, so that this method becomes unpractical.
On the other hand, in FIG. 18 for example, a method not utilizing the row of bottom echoes is shown. In this method, the body 11 to be measured is interposed between a transmitting element 12A and a receiving element 12B of ultrasonic wave, whereby a penetration time difference ti of the ultrasonic wave is measured. In this case, the relationship between the penetration time difference ti and the sonic speed v is shown by the following equations. EQU ti=d/v+(l.sub.1 +l.sub.2)/V.sub.0 ( 6) EQU .DELTA.ti=d.DELTA.v/v.sup.2 ( 7)
Accordingly, when the sonic speed v is to be determined at an accuracy of 1% for example, the sonic speed v=6000 m/s, sheet thickness d=0.3 mm and .DELTA.v/v=0.01 are substituted into the equation (7) to obtain .DELTA.ti=5.times.10.sup.-10 sec. At this time, it is necessary to keep constant a distance between ultrasonic probes 12A and 12B with a permissible fluctuation only 0.75 micrometer. This value is achievable in a research laboratory, however, it is very impractical.